The hyena attacks quickly, with little warning, and always goes through the shield, ending up at the Lancer's back. This forces the Lancer to have to turn around and try to get a quick poke in before the hyena attacks again.
Repeat ad infinitum.
This was a big part of why it took me 5 weeks to reach level 60 - Every single mob becomes a lengthy, drawn out affair that either requires you to suck up the damage and spend time recovering, or block and take 10 minutes per mob.
In the end it's totally worth it to have a level 60 lancer, they are a ton of fun to play. This just explains why, in addition to people just not wanting to play the tank role, you don't see many lancers at cap.